Thermal stability of jet fuel is typically described in terms of the fuel's tendency to form deposits on fuel lines, valves, injectors, and combustion chamber surfaces in engines. These fuel system deposits can be created by two distinct free radical pathways: low temperature autoxidation (usually called fouling) and higher temperature pyrolysis (usually called coking or carbon deposition).
Prolonged operation of gas turbine engines at elevated fuel temperatures (say, above 300° F./149° C.) accelerates the fuels reactions that lead to gum and carbon deposit accumulation on various fuel-immersed components such as: fuel filters (increasing the pressure drop across them and reducing the fuel flow); fuel injector orifices (deforming their spray pattern thus leading to localized high-thermal load areas in combustors—i.e. “hot spots”); FMUs (interfering with proper engine controls operability) heat exchangers (reducing the fuel flow and thus heat transfer capabilities), fuel lines (clogging and partially blocking fuel flow); valves, seals, connectors, actuators, etc. These carbon deposits (which have much lower thermal conductivity than typical heat exchanger surface materials and, hence, lower the heat exchangers' capability to conduct heat during this heat exchanger fouling) may lead to higher maintenance costs, more frequent repair/replacements of fuel-immersed components and ultimately, component failures. In general, fuel system fouling inhibits efficient heat transfer which, in turn, results in higher component temperatures and potentially catastrophic component failures.
Oxidative stability (autoxidation) of jet fuel differs from its thermal stability by referring to the rate at which oxygen is consumed and oxidative products are formed. Autoxidation reactions occur during fuel storage and exposure to high temperature in fuel lines which results in a series of liquid oxidation reactions of alkyl radicals generating hydroperoxides and other oxidized products which are believed to be responsible for solid deposit formation.
Autoxidation lowers the jet fuel's quality from the amount of gaseous oxygen dissolved into fuel which, in turn, lowers the fuel's thermal sink capabilities.
Other organic contaminants such as FAME (Fatty Acid Methyl Esters) that come from bio-diesel fuels and from cross-contamination during refueling of aircraft from ground-based fuel tankers/bowsers can also pose significant fuel quality problems. Additional accumulation of organic acids due to oxidation of fuel hydrocarbons can occur during the handling of fuel. The resulting carboxylic acids are corrosive to some metals and can increase the solubility of these metals in fuel. Any fuel system components which consist of such metals are likely to be eroded and damaged. Organic acids can increase carbon deposition in fuel and can have negative effect on the fuel's thermal stability and the fuel system's components' material compatibility. Some existing techniques for detection of coking/varnishing in aircraft jet fuel systems rely on many temperature sensors placed around the gas turbine engine to predict when fuel degradation will happen. While these methods may be effective, it is would be advantageous to employ a more effective accurate means of early detection and warning of fuel coking in aviation fuels at elevated operational fuel temperatures.